Experiments upon coloured Shadows. 109 
stationary in the same place, the same varieties in the strength 
of the tints of the coloured shadows may be produced merely 
by opening the window-shutter a little more or less, and ren- 
dering the illumination of the paper by the light from without 
stronger or weaker. By either of these means, the coloured 
shadows may be made to pass through all the gradations of 
shade, from the deepest to the lightest, and vice versa ; and it 
is not a little amusing to see shadows, thus glowing with all the 
brilliancy of the purest and most intense prismatic colours, 
then passing suddenly through all the varieties of shade, 
preserving in all the most perfect purity of tint, growing 
stronger and fainter, and vanishing and returning at com- 
mand. 
With respect to the causes of the colours of these shadows, 
there is no doubt but they arise from the different qualities of 
the light by which they are illuminated ; but how they are pro- 
duced, does not appear to me so evident. That the shadow 
corresponding to the beam of daylight, which is illuminated 
by the yellow light of a candle, should be of a yellowish hue, 
is not surprising ; but why is the shadow corresponding to 
the light of the candle, and which is illuminated by no other 
light than the apparently white light of the heavens, blue f I 
at first thought that it might arise from the blueness of the 
sky ; but finding that the broad daylight, reflected from the 
roof of a neighbouring house covered with the whitest new 
fallen snow, produced the same blue colour, and if possible of 
a still more beautiful tint, I was obliged to abandon that 
opinion. 
To ascertain with some degree of precision the real colour 
of the light emitted by a candle, I placed a lighted wax candle, r 
